Comparison Of Fisetin And Quercetin Oxidation With A Cell-Free Extract Of Onion Trimmings And Peel, Plant Waste, Containing Peroxidase Enzyme: A Further Insight Into Flavonol Degradation Mechanism

A peroxidase-active, cell-free extract prepared from onion solid wastes was studied with regard to its biocatalytic properties, using fisetin (3,3', 4', 7-tetrahydroxyflavone) as substrate. This particular substrate was selected to investigate specifically the role of the A-ring of the flavonol skeleton in the oxidisability and cleavage pathway(s), and ascertain whether changes in substitution pattern may affect product formation, compared with quercetin (3,3', 4', 5,7-pentahydroxyflavone). Oxidation was shown to be optimum at pH values between 5 and 7 and temperatures between 20 and 40 degrees C. HPLC analyses of a fisetin solution treated with the cell-free extract revealed the formation of three major products, including 3,4-dihydroxybenzoic acid (protocatechuic acid), 2,4-dihydroxybenzoic acid (beta-resorcylic acid) and the depside 2-(3,4-dihydroxybenzyloxy)- 4-dihydroxybenzoic acid. The tentative identification of these substances permitted an approach to a putative oxidation pathway, which appeared to have important similarities with that of quercetin.